The subject matter of the present invention relates generally to improved electrostatic electron lenses, and in particular to improved electrostatic electron lenses to be utilized in electron beam devices including photoelectron microscopes.
In the operation of certain electron microscopes, such as photoelectron emission microscopes, also known as photoelectron microscopes, which may have magnification of 100X to 200,000X, it is necessary that the specimen, the lenses, and other parts be contained in a clean, ultra-high vacuum environment of, for example, from 10.sup.-9 to 10.sup.-11 Torr to avoid specimen contamination. As a result, the entire microscope must be heated before use and held at an elevated temperature sufficiently long to cause any adsorbed gases within the microscope apparatus to be released into the air which are then evacuated from the microscope leaving the interior cavity in a very clean and ultra-high vacuum condition. The temperature utilized may be from 150.degree. C to 300.degree. C. In the past, during this heating process, sometimes referred to as "bake-out", the electrodes of the electrostatic electron lenses would expand and fracture the insulators between such electrodes which constrained their movement, or such electrodes became physically distorted. This was costly and time consuming, inasmuch as the entire microscope system had to be disassembled and the damaged lens removed for repair before operation could resume.
Additionally, in a photoelectron microscope, the specimen to be observed must be illuminated with light, preferably ultraviolet, to cause the specimen to "photo-emit" electrons. In the past, this proved to be a difficult task inasmuch as the light source was best disposed outside of the ultra-high vacuum environment to facilitate access to such source. Also, the specimen holder was, by necessity, positioned in a manner precluding a direct line of sight path between the light source and the specimen. This required that the light be reflected off an appropriate reflecting surface to illuminate the specimen. In known photoelectron microscopes, this has been accomplished in an expensive, inefficient manner by providing the microscope with a special mirror or reflecting surface separate from the electron lens and disposed within the microscope at an appropriate location to reflect light onto the specimen. Problems that are inherent in this past solution to the problem are an undesirably large spacing distance between the specimen and the lens, delicacy of alignment, extra cost and sensitivity to vibration, as well as inefficiency.
The improved electrostatic electron lens of the present invention is an improvement over my previous electron lenses, disclosed in U.S. Pat. No. 2,536,878, which issued Jan. 2, 1951 to Gertrude M. Fleming, and is entitled "ELECTRON LENS;" U.S. Pat. No. 2,740,919, which issued Apr. 3, 1956 to Gertrude M. Fleming and is entitled "ELECTRON LENS," and U.S. Pat. No. 3,253,144, which issued May 24, 1966 to Gertrude F. Rempfer, and is entitled "ELECTRON LENS HAVING MEANS FOR CORRECTING ASTIGMATISM." The present invention is also an improvement over my previous electron lenses utilized in my previous microscope disclosed in U.S. Pat. No. 2,617,041, which issued Nov. 4, 1952 to Gertrude M. Fleming and is entitled "STEREOSCOPIC ELECTRON MICROSCOPE." My previous electron lenses, as well as all of the known art of which I am aware, suffer from the difficulties of specimen illumination problems and lens insulator fracture as hereinbefore described.
In accordance with the present invention, a photoelectron microscope is described having an objective lens, and two projection lenses. These lenses are of the improved electrostatic electron type utilizing my present invention. The lenses focus the electrons emitted by the specimen into a beam and are aligned in axial alignment with one another allowing the electron beam to pass through their central apertures. The objective lens has its beam entrance electrode provided with a planar upper surface having a highly polished mirror finish and is disposed in spatial relationship below the specimen holder. This mirror surface reflects light from appropriate sources onto the specimen for photoemission, or allows visual observation of the specimen by the operator for specimen positioning. The intermediate electrode of each of the lenses is provided with an outer annular shoulder portion circumferentially disposed about its outer periphery, cooperating in an abutting overlapping relationship with an inner shoulder projection on one of the insulators, and disposed radially outward of such one insulator. This enables radial expansion of the intermediate electrode without fracturing the insulative support means or bending such electrode.